1. Field of the Invention
The invention relates to a fuel cell system.
2. Description of the Related Art
In recent years, fuel cells are drawing attention as an electric power source that is excellent in operation efficiency and environmental friendliness. The fuel cells are able to output an electric power commensurate with a demand of load by controlling the amount of supply of a fuel gas. In some cases, however, the output voltage of a fuel cell does not equal the voltage that is demanded by a load. Therefore, a technology that makes the output voltage of a fuel cell equal to the voltage that a load demands, by converting the output voltage of the fuel cell via a DC/DC converter has been proposed (see, e.g., Japanese Patent Application Publication No. 2003-217625 (JP-A-2003-217625)).
The DC/DC converter has an electronic switch, a diode and an inductance as basic elements, and changes voltage through the switching action of the electronic switch. Since ripples occur due to the switching action of the electronic switch, the DC/DC converter is equipped with a snubber circuit that absorbs the ripples. The snubber circuit absorbs ripples by a capacitor. If the charge stored in the capacitor is not utilized, the energy conversion efficiency of the DC/DC converter declines. Therefore, technologies for effective utilization of the charge stored in a capacitor by regeneration have been proposed (see, e.g., Japanese Patent Application Publication No. 2005-143259 (JP-A-2005-143259)).
The DC/DC converter is known to be able to reduce the switching loss by performing soft switching. However, if this is employed to boost the voltage of the fuel cell, there is a possibility that the electric power stored for the soft switching may be input to the fuel cell. Thus, application of the DC/DC converter is difficult. Therefore, a fuel cell system capable of boosting the output voltage of the fuel cell by performing the soft switching which is described in Japanese Patent Application Publication No. 2009-165246 (JP-A-2009-165246) has been proposed.
The fuel cell system described in Japanese Patent Application Publication No. 2009-165246 (JP-A-2009-165246) is equipped with a voltage boost converter. The voltage boost converter has: a main voltage boost portion that has a first switch (constructed of a switch element and a diode) and a first coil, and that boosts the output voltage of the fuel cell by the counter electromotive force that is caused in the first coil by the first switch performing the switching action on the first coil; and a subsidiary voltage boost portion which has a capacitor that adjusts the potential difference between the two poles of the first switch by the amount of electricity stored in the capacitor, and which reduces the switching loss of the first switch by adjusting the stored amount of electricity of the capacitor during the switching action, and which includes a second switch (constructed of a switch element and a diode) and a second coil.
Incidentally, in the voltage boost converter as described above, if both the first switch and the second switch are turned off, reverse current instantaneously flows into the diode of the second switch, and then instantaneously stops. Due to this phenomenon, surge voltage occurs. In order to cope with this phenomenon, the present inventors conducted experiments in an arrangement in which a diode was added to the subsidiary voltage boost portion, and a cathode was connected between the second switch and the second coil, and observed the occurrence of a phenomenon in which surge voltage occurred, and caused overvoltage destruction of the added diode.
For this phenomenon, it is considered useful to add a third coil that serves as a saturable reactor, and it is also conceivable to cause the second coil as a resonance reactor to perform the added saturable reactor function. Thus, a technology in which the resonance reactor is provided with a saturable reactor characteristic has also been proposed (see, e.g., Japanese Patent Application Publication No. 2003-018833 (JP-A-2003-018833)).
Incidentally, Japanese Patent Application Publication No. 2003-018833 (JP-A-2003-018833) describes the resonant reactor as follows. Concretely, the resonant reactor has a winding wire that is a Litz wire (stranded wire), and its core has a toroidal shape. As for the core of the resonant reactor, its outside diameter and inside diameter are substantially equal to those of the core of a flyback transformer, and the thickness thereof is substantially equal to or smaller than that of the core of the flyback transformer. Besides, with regard to the magnetization characteristic of the core of the resonant reactor, the foregoing application describes that the core is unsaturated when the electric current flowing the resonant reactor is within a predetermined range, and that the inductance value thereof is substantially constant in such a region, and that in a region in which the electric current is above the predetermined range, the core is saturated and the inductance value decreases.
Thus, Japanese Patent Application Publication No. 2003-018833 (JP-A-2003-018833) describes the function or the like of a resonant reactor that has saturability characteristic, but does not clearly describe the construction thereof. Thus, according to this patent application, it is unclear what type of saturable reactor is suitable for a voltage boost converter of a fuel cell system.